The present invention relates to a simultaneous multi-beam light modulation system, which can be employed in a laser printing apparatus.
In a well-known simultaneous multi-beam light modulation system, a plurality of image signals are assigned to carriers, respectively and the amplitude of each carrier is modulated, whereby a plurality of modulated signals are produced, and at the same time, by the modulated signals, an acoustic optical element is actuated, so that a laser beam is divided and modulated by the acoustic optical element. This system is employed in a laser printing apparatus and is practical for use in lowering the deflection speed of a scanning optical deflection apparatus. In this system however, since the acoustic optical element is actuated simultaneously by a plurality of modulated signals, the light modulation intensity of each image signal is changed under the influence of the other image signals, so that cross modulation occurs between the multiple beams from the acoustic optical element.
Under the circumstances, a system as shown in FIG. 1 has been proposed for the purpose of obviating such cross modulation. In this system, a plurality of image signals from a signal source 1 respectively modulate the amplitude of the carriers assigned to the respective image signals, from high-frequency oscillators 5 to 7 in AM modulators 2 to 4. The polarities of the respective image signals from the signal source 1 are inverted in inverters 8 to 10 and summed up by an adder 11. The output signals from the adder 11 modulates the amplitude of an assigned carrier from a high-frequency oscillator 13 in an AM modulator 12. The output signals from the AM modulators 2 to 4 and 12 are mixed by a mixer 14 and are then amplified by an amplifier 15 so that the amplified signal is applied to an acoustic optical element 16. The acoustic optical element 16 diffracts a laser beam 17 by Bragg diffraction and produces diffracted lights of first order 18 to 21 corresponding to the output signals from the AM modulators 2 to 4 and 12, and a light of zero order 22. Of the multiple diffracted light beams 18 to 21, the light beam 21 which corresponds to an output signal of the AM modulator 12 is cut out by a light cut plate 23, so that only the light beam 18 to 20 corresponding to the image signals are taken out.
In this system, the output signals modulated by the image signals and the output signals modulated by the image signals whose polarities are inversed are applied to the acoustic optical element 16. Therefore, the light modulation intensity by each image signals becomes constant, without being influenced by other image signals, so that cross modulation between the respective beams occurs less. However, since the respective polarities of a plurality of image signals from the signal source 1 are first inversed by the inverters 8 to 10 and are then added by the adder 11, the necessary electric circuits become complicated. Furthermore, there exist as many image signals as the corresponding multiple beams 18 to 20, and the dynamic range of each beam is equal, but the AM modulators 2 to 4, the amplifier 15 and the acoustic optical element 16 have their own frequency bands which are not flat, and the respective beams more or less influence each other. Therefore, the characteristics ratio of image signal to light output are not always identical.